The present invention relates to a choke assembly for controlling the flowrate or pressure of a fluid. The invention relates in particular to a choke assembly for use in the control of flow of oil and/or gas streams, especially in the control of fluid streams produced from subterranean wells. The choke assembly of the present invention is particularly suitable for use in subsea installations for the drilling of subterranean wells and/or the subsea production of oil and gas.
The flowrate and/or pressure of a fluid stream are generally controlled in use by some form of valve assembly, in which the size of the orifice or conduit through which the fluid is caused to pass is altered. A particular form of assembly commonly employed for the control of fluid flowrate and/or pressure is a choke assembly. Known choke assemblies comprise a conduit having a plurality of orifices and a means for progressively opening and closing some or all of the orifices to allow the passage of fluid therethrough. The desired pressure and/or flowrate is determined by the number and size of orifices that are open and available for the passage of fluid.
A common design of choke assembly is of the plug and cage variety. A generally cylindrical cage is disposed within the choke body, the cage being provided with a plurality of openings or orifices therethrough. A cylindrical plug is disposed to be moveable co-axially with respect to the cage, so as to open or close the orifices in the cage, depending upon the position of the plug. The orifices in the cage are disposed along the path of movement of the plug, such that movement of the plug from the fully closed to the fully open position along the longitudinal axis of the cage opens successive orifices, thereby increasing the cross-sectional area available for fluid to flow. Typically, the fluid to be controlled is introduced from the inlet of the choke assembly perpendicular to an annulus surrounding the exterior of the plug and cage, passes through the orifices of the cage into the interior of the cage and, from there to the outlet of the choke assembly. The orifices in the cage are disposed perpendicular to the longitudinal axis of the cage. Typically, equal and opposite orifices are used, to generate jets of fluid entering the interior of the cage to impinge on one another, thereby dissipating the energy in the fluid stream.
In general, known choke assemblies of the aforementioned type have one of two arrangements. In a first arrangement, the cage is fixed and the plug is moveable longitudinally with respect to the cage. The plug extends and is moveable within the cage. In a second arrangement, the cage is again fixed and the plug is moveable, but with the plug disposed externally to the cage (generally known as a sleeve). In general, the arrangement employing an external cage and an internal plug is preferred, as this provides a better degree of control of the fluid flowrate and/or pressure. However, there are several significant drawbacks with both designs of choke, in particular the design employing an external cage.
It is the case that a subterranean well produces a fluid stream having several phases of fluids. Liquid phases present in the fluid stream are typically oil and water. Water is being produced from subterranean wells in increasing quantities, for example as a result of operations to enhance oil recovery from a field by water injection. In addition, the fluid stream produced will typically contain significant volumes of gas.
Much effort is being put into developing systems to separate oil, water and gas from the fluid stream produced by wells. In particular, given the increasing depths at which subsea wellhead installations are operating, it is becoming increasingly desirable to avoid having to produce water from the well to the surface. Rather, there is increasing need to separate water produced from the well at the seabed, to allow for reinjection.
However, conventional choke assemblies provide an obstacle to achieving the desired fluid separation. The conventional plug/cage choke assembly has orifices extending perpendicularly through the cylindrical cage. The orifices are typically circular. As a result of this arrangement, the fluid passing through the choke assembly is subjected to very high rates of shear. This in turn generates significant mixing of the fluid phases, in some cases resulting in emulsification of the oil and water phases. This mixing significantly hinders the separation of the water, oil and gas phases.
Accordingly, there is a need for a choke assembly that provides the required level of fluid control, without subjecting the fluid stream to excessively high rates of shear.
U.S. Pat. No. 6,730,236 relates to a method for separating liquids in a separation system having a flow coalescing apparatus and a separation apparatus. The separation system includes a flow conditioning apparatus having an inlet and an outlet. A swirl chamber is disposed between the inlet and outlet and operates to create a swirling fluid flow pattern. It is suggested that this swirling pattern induces coalescence of liquid droplets in the fluid stream. The flow conditioning apparatus comprises an outer shell in which the fluid inlet is formed. The apparatus further comprises an inner swirl chamber having a helical pattern of tangential holes, whereby fluid enters the swirl chamber from the outer shell and is caused to flow in a helical pattern. The flow of fluid into the inner swirl chamber is controlled by a plunger assembly, including a conical head that is moveable longitudinally within the swirl chamber. Reciprocation of the conical head covers and uncovers holes in the helical pattern and allows the fluid flow to be controlled.
The flow conditioning system of U.S. Pat. No. 6,730,236 is intended for use in conjunction with a downstream apparatus for separating the fluid phases and acts to condition the flow by inducing coalescence of fluid bubbles and, if required, to act as a choke device to control the fluid flowrate. It would be advantageous of a system could be provided that combines the operation of a choke and a separation action. In addition, it would be particularly useful if such a system could act to separate solids from a fluid stream.
Further, the production of oil, water, and/or gas from a subterranean well, it is very often the case that the fluid stream has entrained therein significant quantities of solid material. The solid material, such as sand, silt and gravel, may be produced from the subterranean formations along with the oil and gas. Sand and gravel entrained with the oil and/or gas will enter the choke assembly together with the fluid stream. In addition, the well may produce quantities of metal particles that enter the choke, for example as a result of equipment wear or failure upstream of the choke assembly.
In the designs of choke discussed hereinbefore, the cage assembly is particularly vulnerable to damage from solids entrained with the fluid stream. Small particles entrained with the fluid generates a very high rate of wear on the cage and especially the plug, leading to poor control of the fluid flowrate and/or pressure and eventual failure of the choke. In addition, with the arrangement of perpendicularly extending, opposed orifices in the cage, solid particles and objects entrained with the fluid are directed onto the portion of the interior surface of the cage opposite the orifice. In extreme, but not uncommon cases, large solid particles impacting the cage assembly can destroy the cage. In all such cases, the inevitable result is that the choke assembly requires replacement. In the case of a subsea installation, perhaps at a depth of many thousands of feet of water, the replacement of a choke is a difficult, dangerous and time consuming task, during which production from the well may need to be shutdown. Chokes having the external cage/internal plug design are particularly vulnerable to damage and failure from entrained solids. Choke assemblies located close to the wellhead are particularly vulnerable to solids produced from the well. However, as such choke assemblies are typically operating at or close to wellhead pressure, their failure can lead to a potentially dangerous situation and their replacement is a particularly difficult task, especially at great depths of water.
Accordingly, there is a need for a choke assembly that reduces the damage caused to the cage of the choke assembly by solids entrained with the fluid being fed to the choke.
As noted, choke assemblies are required in order to reduce the pressure and/or flowrate of fluid streams, in particular fluid streams produced from subterranean wells. It has been found that this necessary choke operation can also be employed to provide a fluid separation function and to condition the fluid stream for further downstream processing. In particular, it has been found that the choking operation can be employed to separate solid particles entrained or suspended in fluid streams. Developments have also been made to the choke assembly to prevent such entrained and suspended solid particles from damaging the components of the choke, as hereinbefore described.